Are there any data on water vapor concentrations? I ask that because I have read that the contribution of water vapor to the greenhouse effect is perhaps two to four times greater than CO2.
Water vapor is a much larger contributor to the greenhouse effect than CO2. There is a lot more of it in the atmosphere.
The reason water vapor cannot be a forcing (i.e. root cause) of global warming is because of how short the water cycle is. If the amount of water vapor in the atmosphere increases, it is rather quickly balanced back by fixing some of the water vapor into liquid water. We call this process "precipitation."
I've seen estimates that the length of the water cycle is on the order of a week or two. So if human activity puts a lot of extra water vapor into the air--say, with a nuclear power cooling tower--then the excess water vapor will precipitate back into the liquid water within a week or two.
Human society has adapted very well to this cycle. We have built our societies to manage rain and snow [1].
The carbon cycle is much longer--I've seen estimates on the order of several decades at least. Thus, if human activity puts an excess of CO2 into the atmosphere, that will be there for several decades or longer. Thus the effect has quite a bit longer to compound, producing changes that seem long-term to human societies.
We have not built our societies to adapt to the effects of these types of trends. Think of how many human structures within just a few meters elevation above mean sea level, for example. Like: most of Manhattan.
Water vapor is a feedback. If the average atmospheric temperature goes up (for any reason), it can hold more water vapor, which then further increases the greenhouse effect, which raises the temperature more, etc. One reason scientists build complex computer models is to figure out how a little bump in temperature (from CO2 for example) can be magnified by feedbacks like water vapor.
[1] Well, at least in the volumes that have been common over the past few hundred years.
Not only that, all greenhouse models require a 'water feedback loop'.
Of course there are plenty of anthropogenic water emissions to go about. When you irrigate, you are effectively throwing water into the air - by increasing the surface area of water evaporation through plant transpiration networks.
I have never seen anyone talk about this alternative hypothesis.
A disclaimer - I am all for reducing CO2 emissions strictly on principle (this is not dependent on the effects of CO2) and have put my money behind my beliefs - I have driven hybrid vehicles to support technology towards that end since 2003. But in my judgement as a scientist in general (and to some degree as a chemist who has done advanced spectroscopy including IR spectroscopy) I am losing confidence in climatology.
It's a reasonable question. While I am not a climatologist...
Anthropogenic evaporation is probably minuscule next to deviations in natural evaporation due to small changes in temperature. A single hurricane, for example, dumps more power (as heat) than all human power generation.
Also, unlike CO2, I would also guess that atmospheric water probably reaches equilibrium much faster (half life ~1 year instead of ~30 years).
There is basically no equilibrium condition here. If you mean that there is a more 'inelastic' equilibrium constant, then keep in mind that the concentration level governed by such a constant is dependent on rate in versus rate out, and there are certainly ongoing inputs of ag water. We've completely drained several seas and underground aquifers as a result of human activity in the last 50 years.
Ah, but that's not at all the same as not addressing it, or as you state "I have never seen anyone talk about this alternative hypothesis." (of course I read it, and asked that question)
They are talking about it, and saying that it's a potential source of error that nobody knows how to account for yet, which is pretty common in science. If someone can demonstrate that the potential effect is dominant, it's a real problem. If someone can demonstrate that the potential effect is trivial it can safely be left alone. Otherwise you do the best modeling you can with the best data you can and talk about the possible confounding issues as so we proceed.
So I don't see any particularly damning issue here, have I missed something? It would be nice to resolve the differences between estimates. Is i the lack of analysis of radiative forcing that is bothering you? Do we have any reason to believe it can be a significant term? These sources of vapor are certainly not large (relative to total sources of water vapor in the atmosphere) so I'm not seeing the mechanism.
This really isn't my area, and as always I'm more than happy to be wrong (I learn more that way).
Perhaps the searches are [EDIT: not] looking in the PDFs? I just opened the Summary for Policymakers for the Physical Science Basis report, and found a few discussions of water vapor in the atmosphere.
I'd try looking at "Physical Science Basis" and "Impacts, Adaptation and Vulnerability" reports; try the Summaries for Policymakers first, and the full reports if those don't have what you need (though go straight to the full reports if you have time -- they can be fascinating!).
I opened the "Summary for Policymakers" listed right on their home page, and it popped up a PDF titled "AR5_SYR_FINAL_SPM-1.pdf". That's where I searched on the word "vapor" and found only one occurrence -- inside the word "evaporation".
I guess the "for the Physical Science Basis" report is something different that I haven't found yet.
Holy cow do they make it hard. I had JavaScript off, which made their page much more usable. Either do that or ...
With JavaScript on, apparantly the UX concept is 'Easter eggs': There are 4 images arranged horizontally at the top. If you click image, one the section beneath it changes. The second image is "Physical Science Basis".
All that work making the reports accessible, hamstrung by web design.
The reason water vapor cannot be a forcing (i.e. root cause) of global warming is because of how short the water cycle is. If the amount of water vapor in the atmosphere increases, it is rather quickly balanced back by fixing some of the water vapor into liquid water. We call this process "precipitation."
I've seen estimates that the length of the water cycle is on the order of a week or two. So if human activity puts a lot of extra water vapor into the air--say, with a nuclear power cooling tower--then the excess water vapor will precipitate back into the liquid water within a week or two.
Human society has adapted very well to this cycle. We have built our societies to manage rain and snow [1].
The carbon cycle is much longer--I've seen estimates on the order of several decades at least. Thus, if human activity puts an excess of CO2 into the atmosphere, that will be there for several decades or longer. Thus the effect has quite a bit longer to compound, producing changes that seem long-term to human societies.
We have not built our societies to adapt to the effects of these types of trends. Think of how many human structures within just a few meters elevation above mean sea level, for example. Like: most of Manhattan.
Water vapor is a feedback. If the average atmospheric temperature goes up (for any reason), it can hold more water vapor, which then further increases the greenhouse effect, which raises the temperature more, etc. One reason scientists build complex computer models is to figure out how a little bump in temperature (from CO2 for example) can be magnified by feedbacks like water vapor.
[1] Well, at least in the volumes that have been common over the past few hundred years.